In laying electric conductors on concrete forms prior to embedding in concrete, or in the erection of hollow walls, there is a problem with secure attachment of the conduit or tubes with so-called junction boxes which later serve for the running of cables and which are, as a rule, made of plastic and have a very smooth surface. Secure attachment is necessary in order to prevent a shifting of the tubes when cables are later laid and when concrete is poured in and vibrated. In addition, the connections between tubes and boxes should fit as tightly as possible.
A tool for this purpose has already been made known by the applicant, and can form a chamfer on the end of the tube, and cut a fir-cone-shaped rib into the outer surface of the tube at a distance from the end of the tube so that the raised rib engages the wall of the inlet case and acts as a stop upon partial withdrawal of the tube end out of the through bore or hole cut in the case or box. This prevents the end from being pulled out of the box due to strain on the tube during the pouring of the concrete or the laying of cables, since the fir-cone-shaped rib forms an effective stop. The interaction of hole, tube and rib provides, moreover, a sufficiently tight-fitting seal against the penetration of dampness into the box. Furthermore, the chamfer of the end of the tube facilitates the insertion of the tube into the hole.
Problematic with such a tool is, however, the bore head for making the through bore in the plastic case, which apart from the previously-mentioned inlet case, can also be any other box for electrical installations. These problems include the great danger of injury to the user from the exposed bore head and the fact that it is easily damaged, and the difficulties with definite, centered positioning of the bore head on the smooth wall of the case.